Coyote Lake Research Articles

Spatial distribution of aftershocks and background seismicity in central California

We examine the spatial distribution of earthquake hypocenters in four central California areas: the aftershock zones of the (1) 1984 Morgan Hill (2) 1979 Coyote Lake, and (3) 1983 Coalinga earthquakes, as well as (4) the aseismically creeping area around Hollister. The basic tool we use to analyze these data are frequency distributions of interevent distances between earthquakes. These distributions are evaluated on the basis of their deviation from what would be expected if earthquakes occurred randomly in the study areas. We find that both background seismic activity and aftershocks in the study areas exhibit nonrandom spatial distribution. Two major spatial patterns, clustering at small distances and anomalies at larger distances, are observed depending on tectonic setting. While both patterns are seen in the strike-slip environments along the Calaveras fault (Morgan Hill, Coyote Lake, and Hollister), aftershocks of the Coalinga event (a thrust earthquake) seem to be characterized by clustering only. The spatial distribution of earthquakes in areas gradually decreasing in size does not seem to support the hypothesis of a self-similar distribution over the range of scales studied here, regardless of tectonic setting. Spatial distributions are independent of magnitude for the Coalinga aftershocks, but events in strike-slip environments show increasing clustering with increasing magnitude. Finally, earthquake spatial distributions vary in time showing different patterns before, during, and following the end of aftershock sequences.

Slip partitioning of the Calaveras Fault, California, and prospects for future earthquakes

Examination of main shock and microearthquake data from the Calaveras fault during the last 20 years reveals that main shock hypocenters occur at depths of 8–9 km near the base of the zone of microearthquakes. The spatial pattern of pre‐main shock microseismicity surrounding the Coyote Lake and Morgan Hill hypocenters is similar to the pattern of the post‐main shock microseismicity. Microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones. Estimates of the fault regions which slipped during the Coyote Lake and Morgan Hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic. We propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes. From the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the Calaveras fault. From an analysis of historic seismic data we establish the main shock rupture history for each aseismic zone and identify two zones that are the most likely sites for the next M > 5 earthquakes. The first zone is located near Gilroy and was last ruptured by a M5.2 earthquake in 1949. The second zone is located south of Calaveras Reservoir and north of the 1988 M5.1 Alum Rock earthquake. It has not slipped seismically since at least 1903, and the size of the aseismic region is sufficiently large to sustain a M5.5 earthquake.

Spatial patterns of aftershocks of shallow focus earthquakes in California and implications for deep focus earthquakes

Previous workers have pioneered statistical techniques to study the spatial distribution of aftershocks with respect to the focal mechanism of the main shock. Application of these techniques to deep focus earthquakes failed to show clustering of aftershocks near the nodal planes of the main shocks. To better understand the behavior of these statistics, this study applies them to the aftershocks of six large shallow focus earthquakes in California (August 6, 1979, Coyote Lake; May 2, 1983, Coalinga; April 24, 1984, Morgan Hill; August 4, 1985, Kettleman Hills; July 8, 1986, North Palm Springs; and October 1, 1987, Whittier Narrows). The large number of aftershocks accurately located by dense local networks allows us to treat these aftershock sequences individually instead of combining them, as was done for the deep earthquakes. The results for individual sequences show significant clustering about the closest nodal plane and the strike direction for five of the sequences and about the presumed fault plane for all six sequences. This implies that the previously developed method does work properly. Nonrandom behavior was also found about the slip directions, the P axis, the T axis, and the B axis, but this is probably caused by the lack of independence between these axes and the previously mentioned features of the focal mechanisms. Given that the method does work and that deep aftershocks were not shown to cluster about the main shock nodal planes, the shallow focus data were used to simulate the deep focus study. The goal is to determine if there are artificial factors that make clustering in the deep focus data unobservable. To more closely mimic the work on deep earthquakes, the largest aftershocks from each of the six sequences were combined and studied with respect to their respective main shock focal mechanisms. This reduced the significance of the clustering about the focal mechanism parameters, but not below 95% confidence. Gaussian noise was then added to the aftershock hypocenters in order to determine if the larger hypocentral and focal mechanism errors in the deep focus data could account for the previous negative result. The conclusion is that the following reasons are sufficient to explain the lack of clustering about the main shock nodal planes for the deep focus aftershocks: the need to combine aftershocks from several sequences, the size of the hypocentral location and focal mechanism errors, and the alignment of distant aftershocks with the Wadati‐Benioff zone.

Temporal variation of body-wave attenuation using earthquake doublets

Summary. A technique to measure temporal variations of P and S body-wave attenuation is presented. Spectral ratios of earthquake doublets, recorded in the same station at different dates, are computed. Doublets are pairs of earthquakes which share the same hypocentre and magnitude; they have nearly identical wave forms and can be compared by using signal processing techniques. Three doublets were selected in the vicinity of the Coyote Lake, California, earthquake (1979 August 6, M = 5.9). The doublet following the Coyote Lake earthquake does not show any variation in spectral ratio. The doublet preceding the main shock by more than one year shows a small change in P spectral ratios and no change in S. The largest variation, in both P and S spectral ratios, is observed for the doublet whose first event occurred two months before the Coyote Lake earthquake. Despite the difficulty in completely eliminating source and calibration effects, the simplest interpretation of our data is to invoke temporal changes in P and S attenuation. P attenuation was significantly higher (by 8 per cent) and S attenuation larger (by 2-3 per cent), two months before the M = 5.9 shock than one year before and one month after. Such observations remind us of the possible role of the migration of fluids in the crust prior to an impending large earthquake and supports Aki’s plea for attenuation studies as a tool for earthquake prediction.

Analysis of the observed seismic response of a highway bridge

AbstractStrong‐motion accelerograms obtained on the San Juan Bautista 156/101 Separation Bridge during the 6 August 1979 Coyote Lake, California, earthquake are used to examine the response of this multiple‐span bridge to moderate levels of earthquake loading. Although the bridge was not damaged, the records are of significant engineering interest as they are the first to be recorded on a highway bridge structure in North America. A technique of system identification is used to determine optimal modal parameters for linear models which can closely replicate the observed time‐domain seismic response of the bridge. Time variations in frequency and damping in the horizontal response are identified using a moving‐window analysis. A three‐dimensional finite element model is developed to study the bridge response in detail. The first two horizontal modal frequencies computed from this model are in excellent agreement with information obtained during the system identification analysis provided the finite element model's expansion joints are locked, preventing relative translational motions from occurring across the joints. Locking is confirmed by the observed seismic deformations of the structure in the fundamental mode. Fundamental vertical frequencies of the individual spans, predicted by the finite element model, are in very good agreement with ambient vibration test data.

Spatial variation of ground motion determined from accelerograms recorded on a highway bridge

AbstractA set of time-synchronized strong-motion accelerograms, obtained on the San Juan Bautista 156/101 Separation Bridge in California during the 6 August 1979 Coyote Lake earthquake (ML = 5.9), are used to study the spatial variation of ground motion at the bridge site, including traveling wave effects and the influence of multiple-support excitation. Analysis of the ground motion recorded at the base of two of the bridge supports (32.6 m apart) revealed the presence of a differential support excitation having a period of ≈ 3 sec, much longer than any structural periods of the bridge. This signal also appeared as a noticeable long-period component in the superstructure displacements. Analysis of the vertical and radial components of the 3-sec ground motion indicated that ground displacements were retrograde for the duration of strong shaking, with several cycles exhibiting elliptical particle motions. These findings suggest that long-period differential support motions were induced by phase delays in a Rayleigh wave traveling across the bridge site. Further support to this premise is given by the location of the bridge site near a maxima of the Rayleigh wave radiation pattern for the Coyote Lake earthquake (based on published focal mechanism data). Traveling wave effects were also detected for compressional body waves by a correlation analysis which indicated a time delay of ≈ 7 msec between P-wave arrivals at two of the bridge supports.

The Morgan Hill Earthquake of April 24, 1984—Strong-Motion Records

Seventy-two strong-motion accelerograph stations, mostly from within the permanent networks of the U. S. Geological Survey and the California Division of Mines and Geology, were triggered during the Morgan Hill earthquake, April 24, 1984. Among the more interesting of the records are the large acceleration (1.29g) at Coyote Lake dam, the Gilroy array spanning the region between the San Andreas and Calaveras faults, the record from Hall's Valley (epicentral distance 4 km), and records from buildings, a bridge, and a dam within 20 km of the epicenter. Digitized data from 16 stations are available on tape.

The Morgan Hill Earthquake of April 24, 1984—The 1.29g Acceleration at Coyote Lake Dam: Due to Directivity, a Double Event, or Both?

The 1984 Halls Valley (Morgan Hill, California) earthquake had a complex seismic source. Velocities of the major seismic phases measured from continuous broadband seismograms at Berkeley Seismographic Station (BKS) and Richmond Field Station (RFS) show unambiguously that the earthquake is predominantly a double event with the second source hypocenter located approximately 17 km southeast of the mainshock hypocenter given by Bolt, Uhrhammer and Darragh (1985). The southeasterly fault rupture of the first source and the location of the focus of the second source have critical implications for the observed spatial variation of the recorded accelerograms. Of particular engineering interest, the high frequency 1.29g pulse of horizontal ground acceleration measured at Coyote Lake dam can be explained primarily as due to the second source and constructive interference of the principal S waves from the two sources.

Magnitudes and moments of duration

AbstractCoda-duration τ at 42 of the stations in the U.S. Geological Survey's central California seismic network (CALNET) for earthquakes in five source regions of central California—the Parkfield and San Juan Bautista sections of the San Andreas fault, the Sargent fault, the Coyote Lake section of the Calaveras fault, and the Livermore area—are used to obtain empirical formulas relating local magnitude ML and seismic moment M0 to τ and epicentral distance Δ. Models with log2 τ fit the data better than those assuming a log τ dependence. For 55 earthquakes with 1.1. ≦ ML ≦ 5.3, ML = 0.92 + 0.607 (±0.005)log2 τ + 0.00268(± 0.00012)Δ. These ML assume a Wood-Anderson seismograph magnification of 2800; 0.15 should be subtracted from these ML for continuity with magnitudes obtained from or calibrated against typical (magnification ∼ 2000) Wood-Anderson seismographs. For 53 earthquakes with 18.4 ≦ log M0 ≦ 22.3, log M0 = 17.97 + 0.719(± 0.0007)log2 τ + 0.00319(±0.00013)Δ. These relations provide unbiased estimates of ML for 1.5 ≲ ML ≲ 5.3 and 19 ≲ log M0 ≲ 22.3. Station corrections can significantly improve the accuracy and precision of ML and log M0 estimates, particularly if τ from a small number of stations are used. Regional variations in station corrections reflect an increase in coda duration toward the south within the CALNET.

Dynamic analysis of a slope failure from the 6 August 1979 Coyote Lake, California, earthquake : Wilson, R C; Keefer, D K Bull Seismol Soc AmV73, N3, June 1983, P863–877

Dynamic analysis of a slope failure from the 6 August 1979 Coyote Lake, California, earthquake : Wilson, R C; Keefer, D K Bull Seismol Soc AmV73, N3, June 1983, P863–877

Amplification of ground motion and waveform complexity in fault zones: examples from the San Andreas and Calaveras Faults

P-wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of Gaussian beams for point sources located in a low-velocity wedge surrounding a fault. The computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the San Andreas and Hayward-Calaveras faults. Calculations in these models show that the 10–20Hz vertical displacements of earthquakes located at 3–10km depth are amplified by up to an order of magnitude in a 1–2km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust. This amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike-slip earthquake. Depending on the source depth of the earthquake and the structure and velocity contrast of the wedge, multiple triplications in the travel-time curve of direct P- and S-waves will occur at stations in the fault zone. A wedge model successfully predicts the triplications observed in the P waveforms of aftershocks of the Coyote Lake earthquake recorded in the fault zone, showing that body waves from microearthquakes can be used to determine the three-dimensional velocity structure of the fault zone. The amplification, waveform complexity, and distortion of ray paths introduced by the low- velocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone. For realistic models of the wedge, asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20Hz. Numerical methods may be necessary to calculate accurately the wavefield at lower frequencies.

Monitoring velocity variations in the crust using earthquake doublets: An application to the Calaveras Fault, California

We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal. A moving‐window analysis of the entire seismograms, including the coda, gives δ(t), the difference in arrival times versus running time along the seismogram. The time resolution of the method is an order of magnitude better than the digitization interval. The δ(t) technique is illustrated with a pair of microearthquakes, M = 1.7 and 2.0, that occurred before and after the Coyote Lake, California, earthquake (M = 5.9) of August 6, 1979, and on the same segment of the Calaveras fault that ruptured during the earthquake. The coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet, so that its seismogram appears as a stretched version of the earlier event. We interpret this systematic variation in δ(t) along the coda as a change in the average S velocity in the upper crust in the time interval between the two doublets. S wave velocities appear to have decreased by 0.2% in an oblong region 5–10 km in radius at the south end of the aftershock zone.

A new asymptotic method for the modeling of near-field accelerograms

AbstractWe study high-frequency radiation from a dislocation model of rupture propagation at the earthquake source. We demonstrate that in this case all the radiation emanates from the rupture front and, by a change of variables, that at any instant of time the high-frequency waves reaching an observer come from a line on the fault plane that we call isochrone. An asymptotic approximation to near-source velocity and acceleration is obtained that involves a simple integration along the isochrones for every time step. It is shown that wave front discontinuities (critical or stopping phases) are radiated every time an isochrone becomes tangent to a barrier. This leads to what we call the critical ray approximation which is given in a closed form. The previous results are compared with discrete wavenumber synthetics obtained by Bouchon (1982) for the Gilroy 6 recording of the Coyote Lake earthquake of 1980. The fit between the asymptotic and full numerical method is extremely good. The critical ray approximation permits the identification of different phases in Bouchon's synthetics and the prediction of the behavior of the signal in the vicinity of their arrival time.

Seismic moments, local magnitudes, and coda-duration magnitudes for earthquakes in central California

AbstractOnscale seismograms recorded at stations in the U.S. Geological Survey's (USGS) central California seismic network (CALNET) have been used to estimate the seismic moment M0 and local magnitude ML for earthquakes of 1 ≦ ML ≦ 4 located on the San Juan Bautista and Parkfield sections of the San Andreas fault, the Coyote Lake section of the Calaveras fault, the Sargent fault, and near Livermore. These data, together with M0 and ML estimates for 4 ≦ ML ≦ 6 earthquakes in these areas, cannot be fit with a single linear log M0-versus-ML relation. Rather, the data are consistent with log M0 = 1.5 ML + 16 for 3 ≲ ML ≲ 6, with log M0 = 1.2 ML + 17 for 1 1/2 ≲ ML ≲ 3 1/2 and with a slope of ⅔ to 1 fro 1/2 ≲ ML ≲ 1 1/2.Whereas USGS coda duration magnitude MD is consistent with ML for 1 1/2 ≲ ML ≲ 3¼, MD is larger than ML at ML ≲ 1 1/2 and smaller than ML at ML ≳ 3¼. Log M0 can be estimated to a precision of 0.2 for 1 ≦ MD ≦ 3 1/2 earthquakes in central California by applying log M0 = 1.2 MD + 17 to the MD that have been routinely published by the USGS.

841074 Near-source ground motion of the 6 August 1979 Coyote Lake, California, earthquake : Liu, H L; Helmberger, D V Bull Seismol Soc AmV73, N1, Feb 1983, P201–218

841074 Near-source ground motion of the 6 August 1979 Coyote Lake, California, earthquake : Liu, H L; Helmberger, D V Bull Seismol Soc AmV73, N1, Feb 1983, P201–218

Earthquake locations and three‐dimensional crustal structure in the Coyote Lake Area, central California

Previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure. Approximate ray tracing and parameter separation are important elements of the improved method. Application of the method to P wave arrival time data recorded by stations of the U.S. Geological Survey Central California Network yields a three‐dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the Coyote Lake earthquake of August 1979. Very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed. Improved estimates of the locations of earthquakes in the study area are also determined. The relocation of explosions indicates epicentral accuracies of the order of a kilometer or better. Based on the revised hypocentral locations, it is concluded that the San Andreas fault is vertical in this area, with no actual offset between the epicenters and the fault trace. In contrast, the Calaveras has two (or more) active fault surfaces, one nearly vertical and another dipping 75° to the northeast.

Dynamic analysis of a slope failure from the 6 August 1979 Coyote Lake, California, earthquake

abstract The Coyote Lake, California, earthquake of 6 August 1979 (ML = 5.7) provided a rare opportunity to perform a dynamic numerical analysis of a seismically induced slope failure using actual strong-motion records. The earthquake displaced a preexisting slump on the northeast shore of Lake Anderson and formed a fissure 20 m long; offsets of 9 mm vertically and 18 mm horizontally were measured across the fissure. The slump is 9 km northwest of the located epicenter but within 100 m of the mapped trace of the Calaveras fault. Two strong-motion records were written in the vicinity of the slump: Coyote Creek, 5 km to the southeast, and Gilroy #6, 15 km to the southeast. These records combined with field measurements of the slope, estimates of the material properties, and a dynamic numerical model based on the Newmark analysis of seismic slope stability to calculate a predicted displacement for the landslide. A displacement of 27 mm, calculated using the N50°E component of the Gilroy #6 record, is in excellent agreement with the displacement measured in the field (21 mm). However, displacements of only 0.12 mm, calculated using the 250° component of the Coyote Creek strong-motion record, reflect significant local variations in seismic shaking intensity. Because strong-motion records and slope failures rarely coincide, this landslide provides an important test case for our understanding of the dynamics of seismically induced landslides, as well as a warning that local seismologic variations need to be considered in any design applications.

The near-source ground motion of the 6 August 1979 Coyote Lake, California, earthquake

abstractA finite fault striking N24°W and extending to a depth of 10 km is proposed to explain the strong ground motion data for the 6 August 1979 Coyote Lake, California, earthquake (ML = 5.9). Our source model suggests that right-lateral faulting initiated at a depth of 8 km and ruptured toward the south with a velocity of 2.8 km/sec. This unilateral rupture can explain the large displacement recorded south of the epicenter. However, the waveform coherency across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km. The maximum dislocation is about 120 cm in a small area near the hypocenter, and the total moment is estimated to be 3.5 ×1024 dyne-cm. An abrupt stopping phase which corresponds to a deceleration of right-lateral motion can explain the high peak acceleration recorded at array station 6. The stress drop in the hypocentral area is about 140 bars; the average stress drop over the entire rupture surface is 30 bars. The preferred finite-source model can predict the Pn1 waveforms and the beginning features in the teleseismic seismograms. No clear arrivals can be observed in the near-source data for the possible second and third smaller events suggested by Nabelek (personal communication).

Aftershocks of the Coyote Lake, California, earthquake of August 6, 1979: A detailed study

Aftershock hypocenters and focal mechanism solutions for the Coyote Lake, California, earthquake reveal a geometrically complex fault structure, consisting of multiple slip surfaces. The faulting surface principally consists of two right stepping en echelon, northwest trending, partially overlapping, nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 Parkfield, California, earthquake. The overlap occurs near a prominent bend in the surface trace of the Calaveras fault at San Felipe Lake. Slip during the main rupture, as inferred from the distribution of early aftershocks, appears to have been confined to a 14‐km portion of the northeastern sheet between 4‐ and 10‐km depth. Focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex, with left stepping imbricate structure. Seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock. Aftershocks in this zone define a single vertical plane 8 km long between 3‐ and 7‐km depth. Within the overlap zone between the two main slip surfaces, the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone, in close agreement with the stress perturbations predicted by crack interaction models. Aftershock activity in the overlap zone is not associated with a simple dislocation surface. Space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone. This alternation is consistent with local stress perturbations predicted by crack interaction models. We conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which, in turn, control the displacements.Table 5 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐006; $1.00. Payment must accompany order.

Upper bounds on near-source peak ground motion based on a model of inhomogeneous faulting

abstractTwo independent arguments indicate an upper bound of about 10 for the ratio ro/ri in the expressions for peak velocity v and peak acceleration a at close hypocentral distances R: v = (βΔτro/μR)[0.10(ro/ri) + 0.15] and a = (Δτ/ρR)[0.30(ro/ri)2 + 0.45], where ri is the radius of the most heavily loaded asperity that fails within an earthquake source region of radius ro, Δτ is the stress drop, β is the shear-wave velocity, μ is the modulus of rigidity, and ρ is the density; these relationships are for ground motion recorded in a whole-space. First, a recently reported data set was augmented by observations for six earthquakes in the magnitude range 4 ≦ ML ≦ 6.6, for which ground motion was recorded at a minimum of five sites at hypocentral distances of the order of 10 km; the new events include the 1979 Coyote Lake and 1979 Imperial Valley shocks. The entire data set of 22 events, spanning a range in seismic moment from 5 × 1016 to over 1026 dyne-cm, is consistent both with the bound ro/ri < 10 and with the previous conclusion that this ratio does not depend systematically on earthquake size. Second, a theoretical argument, using the result of Savage and Wood that the apparent stress acting on the earthquake fault plane is less than half of the stress drop, is made to the effect that ro/ri < 10. In addition, absolute limits, independent of earthquake size, for peak acceleration are related to the state of stress in the crust; for an extensional state of stress a ≦ 0.40 g and for a compressional stress state a ≦ 2.0 g, where a now represents the maximum horizontal acceleration as recorded at the surface directly above the seismic source.